The ideal of tracing is "seeing every request." But when ten thousand requests come in per second, tracing all of them produces trace data at the same scale as main service traffic. Storage cost, transmission bandwidth, and indexing load balloon to match the workload. That's why sampling, from the very first day of distributed tracing — Dapper's paper already established this — has not been optional but essential. The question is "how to sample." If you naively pick only 1%, payment endpoints with low traffic might capture one trace per hour, if at all. If you pick 100%, noise like health checks eats tracing costs